The present invention relates to lighting applications, more specifically to light emitting diode (LED) and laser diode (LD) applications. More specifically, the present invention relates to a homoepitaxial LED or LD structure and a method for making such a structure.
During the past decade there has been tremendous interest in gallium nitride (GaN) based optoelectronic devices, including light emitting diodes and laser diodes. Because high-quality GaN substrates have not been available, virtually all of the art has involved heteroepitaxial deposition of GaN and GaInAlN on sapphire or SiC substrates. A thin low-temperature buffer layer, typically AlN or GaN, is used in order to accommodate the lattice mismatch between GaN and the substrate and maintain an epitaxial relationship to the substrate.
The use of sapphire substrates with a buffer layer has a number of important limitations for manufacture of LEDs. Sapphire is an electrical insulator, forcing electrical contacts to be made above and to the side of the device structure, rather than above and below (a so-called vertical device structure), wasting space on the wafer. In addition, sapphire has a rather poor thermal conductivity, limiting heat dissipation. Sapphire has a large (16%) lattice mismatch with respect to GaN, so that even with the use of buffer layers a very high level of threading dislocations (107-1011 cm−2) are generated within the device structure. These dislocations may limit performance in certain applications. Deposition of the buffer layer also adds cost and complexity to the process. Sapphire also has a large (45%) mismatch in the thermal expansion coefficient with respect to GaN, which generates stresses in device structures upon cooldown from the processing temperature and limits the maximum size of wafers that can be used without forming cracks. Facets must be prepared at the ends of laser diode structures in order to define the laser cavity, and the difficulty in cleaving c-axis-oriented sapphire makes facet preparation more expensive.
The use of SiC substrates alleviates some of these limitations but introduces other problems. The lattice mismatch to GaN is smaller than that of sapphire, but very high defect concentrations are still generated, and the use of buffer films is still needed. SiC is also much more expensive than sapphire. Lower cost SiC is typically opaque, decreasing the efficiency of the LED device because light emitted from the active region toward the substrate would be absorbed rather than transmitted. Since some applications of LEDs involve emission of ultraviolet light; this light could be absorbed by even a high-quality, transparent SiC substrate because the bandgap is less than that of sapphire or GaN. Moreover, LEDs fabricated on SiC do not generally perform as well as LEDs fabricated on sapphire.
A high-quality GaN substrate would reduce these problems. The substrate could be made electrically conductive as well as semi-insulating, so vertical LED or LD structures could be fabricated. The thermal conductivity of pure GaN is five times that of sapphire, improving heat dissipation, enabling higher power levels, and improving lifetime. Also, there would be no thermal expansion mismatch, resulting in ease of scalability to larger substrates, which would reduce cost. The concentration of threading dislocations would be reduced by 3-10 orders of magnitude, which would reduce leakage currents, improve device yields, and the consistency of I-V characteristics, increase device lifetimes, particularly at high power levels, and may also improve emission efficiency and resistance to static discharge. Furthermore, GaN is much easier to cleave than sapphire, and LD facets can be prepared by simple cleavage rather than by reactive ion etching, further reducing costs.
Some limited work has previously been carried out on forming homoepitaxial LED or LD devices on GaN substrates. Writing in the Journal of Crystal Growth, Pelzmann, et al. reported that homoepitaxial homojunction GaN LED devices demonstrated a doubling of the emission intensity relative to the analogous device on a sapphire substrate. However, homojunction GaN LEDs have much lower emission intensities than InGaN/GaN heterojunction LEDs, as is well known in the art. Therefore, the devices demonstrated by Pelzmann, et al. do not offer any performance advantages relative to conventional heteroepitaxial LEDs.
U.S. Pat. No. 5,637,531 teaches a method for the formation of a GaN crystal which could have applications for homoepitaxial LED formation. This crystal is actually a multi-layer crystal, the fabrication of which requires multiple steps. In addition, U.S. Pat. Nos. 5,770,887 and 5,810,925 to Tadatomo, et al., teach the growth of double-heterostructure LEDs on GaN pseudo-substrates. These pseudo-substrates comprise GaN/ZnO multilayers rather than GaN single crystals. The ZnO served as buffer layers throughout the crystal growth process, and the process therefore required extra steps for the formation and later removal of the ZnO layers.
Kamp, et al. have developed a method for the formation of GaN crystals with homoepitaxial growth thereon. This work, reported in the MRS Internet Journal of Nitride Semiconductor Research, focuses on the application of chemically assisted ion beam etching as a method for polishing the GaN crystal prior to LED formation. The homoepitaxial GaN-based LEDs described by Kamp et al. suffer from a number of important limitations that are overcome by the present invention. The single-crystal GaN substrates were grown in molten Ga with a N2 overpressure of 10-20 kbar at a temperature below 1600° C. The undoped GaN crystals grown by this method have a high concentration of n-type defects, ca. 5×1019 cm−3, believed to comprise oxygen impurities and nitrogen vacancies. As a consequence, the crystals are relatively opaque, with an absorption coefficient of about 200 cm−1 at wavelengths in the visible portion of the spectrum, and up to half the light emitted by the LED is absorbed by the substrate. This constitutes a large disadvantage compared to conventional heteroepitaxial LEDs fabricated on sapphire or transparent SiC substrates. In addition, the substrates employed by Kamp et al. have a dislocation density of approximately 103 to 105 cm−2. This value is lower than the corresponding values in heteroepitaxial LEDs of approximately 107 to 1010 cm−2. Further, the high concentration of n-type defects in undoped crystals grown in molten Ga causes the lattice constant to increase by about 0.01-0.02%, which generates strain in undoped epitaxial GaN layers deposited thereupon. Additionally, the undoped GaN substrates employed by Kamp et al. have a rather limited carrier mobility, about 30-90 cm2 V-s, which may be limiting in high-power devices.
Porowski, et. al., writing in Acta Physica Polonica A, disclosed a method for growing transparent GaN crystals involving the addition of 0.1-0.5% magnesium to a gallium growth medium at temperatures of 1400-1700° C. and nitrogen pressures of 10-20 kbar. This method is capable of producing GaN crystals with an optical absorption coefficient below 100 cm−1. However, these crystals have an electrical resistivity of 104-106 Ω-cm at room temperature, rendering them unsuitable as substrates for vertical light-emitting structures of one type described in the present invention.
It would therefore be desirable to develop a method for forming a high quality GaN substrate, on which to form homoepitaxial LED devices, which would eliminate the above-mentioned problems.